iTunes Store songs are in the AAC file format, not MP3. While this arguably standard/non-standard file type is considered by some to be proprietary to Apple and/or the iTunes Store, this is entirely not the case. Plenty of other software and DAPs (Digital Audio Players) support the AAC format, even including, as John Gruber points out, Microsoft's own iPod competitor, the Zune. If these non-Apple products don't support AAC, they easily could with a software plugin or firmware upgrade.
More important, however, is the fact that the iTunes Store is still the world's most popular legitimate digital download store, and that popularity could skyrocket even farther once they unleash these much, much higher quality downloads (higher than any digital store I know of) in a legitimate, affordable and DRM-free download. The newfound interoperability that Apple is boasting for its present and soon-to-be customers could meet a roadblock, however, if the company doesn't build compatibility for non-iPod devices into the iTunes software itself. While customers can buy DRM-free AAC files through the iTS, iTunes is still the gateway for easily moving those files from one's computer to a DAP, and the claims of interoperability and unparalleled experience could crumble quickly as consumers plug in their Creative Zens, Microsoft Zunes and [insert non-iPod here], only to find out that iTunes scoffs at their non-Apple-branded device.
Will Apple support other DAPs in iTunes? Could we see a 'sync with TiVo' option in the preferences of a forthcoming iTunes 7.5? Time will only tell, but Apple and EMI just opened the doors on the issues of DRM and interoperability - we'll just have to see whether consumers who own something other than an iPod are actually invited to the party.